In recent years, power supply systems using renewable energy such as sunlight and wind power have been researched. Such power supply systems each generate power using renewable energy such as sunlight and wind power in a facility, for example, a building, a factory, or a household. In each facility, there are installed one or more storage batteries which store generated power, and a cooperative ECU (Electronic Control Unit) which controls charging and discharging of the storage batteries. Various electric appliances such as a refrigerator, a television, and an electric vehicle installed in each facility can be operated utilizing the power stored in a plurality of storage batteries.
However, in a power supply system, an increased variation in the frequencies of charging and discharging to and from the storage batteries causes a variation in the power that can be charged to the storage batteries. A storage battery cannot be used once rechargeable power amount of the storage battery falls below a certain value. Therefore, a variation in the life spans of the storage batteries occurs.
For example, when one of a plurality of storage batteries becomes unusable, the remaining usable storage batteries tend to have an increased load. Consequently, the remaining storage batteries also become unusable one after another. Thus, there is a problem in that a deterioration of the entire storage batteries proceeds in this manner.
Conventionally, there has been known a method in which a cooperative ECU detects a temperature and a voltage of each storage battery as an indicator of the quantity-of-state of the storage battery in order to reduce a variation in the life spans of the storage batteries by making the remaining capacities thereof uniform. The remaining capacity of each storage battery can be estimated by detecting a temperature and a voltage of the storage battery. When a storage battery is charged or discharged, a cooperative ECU detects a temperature and a voltage of the storage battery so as to calculate the remaining capacity thereof. The distribution ratio of the power that is charged or discharged to or from each storage battery is calculated in accordance with the ratio of the remaining capacity of the storage battery. For example, when a storage battery is discharged, large power is discharged from a storage battery with a large remaining capacity, and conversely, small power is discharged from a storage battery with a small remaining capacity in accordance with the ratio of the remaining capacity. Thus, a variation in the remaining capacities of the storage batteries can be reduced by calculating a distribution ratio of the power that is charged or discharged to or from each storage battery in accordance with the ratio of the remaining capacity (for example, Patent Literature 1).